There are various processes in use today to produce photochromic ophthalmic lenses—lenses that have the ability to darken when exposed to certain wavelengths of light (such as are contained in sunlight). Among the most common processes are those which imbibe photochromic dyes into the surface of a receptive lens substrate, wherein the prior art describes the dye permeating to a depth of approximately 50 microns (e.g., U.S. Pat. Nos. 5,021,196; 5,130,353; 5,185,390; and 5,882,556). Another type of process directly incorporates the photochromic dyes into a polymerizable casting composition which is then suitably cured to form an ophthalmic lens. Usable photochromic compounds and compositions are covered by numerous patents. Still another process surface casts and cures a polymerizable composition containing photochromic dyes between a front surface mold and a hard lens to form a composite lens (U.S. Pat. No. 5,531,940). Yet another process involves coating a hard lens with a polymerizable coating containing photochromic compounds (e.g., as disclosed in the specification of U.S. Pat. No. 4,756,973).
Other patents, such as this Assignee's U.S. Pat. Nos. 4,758,448 and 4,544,572, disclose the formulation of an abrasion resistant outer coating by gelling a thin (less than about 50 microns) coating material on the surface of a mold and subsequently casting a lens-forming composition onto it, and curing the lens-forming material, wherein the abrasion-resistant outer coating material is not suitable (too hard) for incorporating photochromic compounds.
Each of these processes present certain unique problems. The photochromic performance of lenses which have relatively thin photochromic layers, such as those made by coating (a few microns thick) or by imbibing (perhaps a hundred microns thick) generally degrade faster then those made with the photochromic dyes incorporated throughout the lens.
Lenses made with the photochromic dye incorporated throughout the lens tend to be darker in the unactivated (faded) state due to the inherent color of the dye and the thickness of the lens body. Additionally, color density variations due to thickness variations in an ophthalmic lens prepared to prescription (especially strong prescriptions) can be objectionable in both the activated (darkened) state and the faded state. These lenses can be quite expensive to produce because of the high cost of the photochromic dyes, and when used to make a semifinished lens blank, a considerable amount of dye is wasted in producing a finished prescription lens during the grinding process.
Precautions must be taken when producing composite lenses by surface casting, as disclosed in Gupta et al. U.S. Pat. No. 5,531,940, to ensure good adhesion between the polymerized photochromic layer and the hard lens it is attached to. Such precautions include certain surface modifications to the lens preform as disclosed in Blum et al., U.S. Pat. No. 5,316,702. If conditions are not correct, delamination of the layers can occur.
Composite lenses made by such surface casting of a polymerizable composition onto a hard lens, such as a preform or semifinished lens, suffer from relatively weak bonding between the cast layer and the hard lens. Further, as stated in U.S. Pat. No. 5,405,557, composite lenses manufactured by partially curing one layer of a polymerizable lens-forming composition, and then filling the mold with a second polymerizable lens-forming composition prior to curing the two layers together avoids weakness between the two layers only when the two layers are sufficiently compatible that the fully cured, two layer lens is homogeneous and does not contain a distinct phase boundary between the layers. In accordance with the present invention, using two layers of different polymerizable monomers, oligomers, or polymers, which result in a distinct phase boundary between adjacent, fully cured layers, results in little to no weakness between the layers such that delamination does not occur.
The front and rear layers of the composite lenses manufactured in accordance with the present invention are different in chemical composition and will also be different with respect to other physical properties such as index of refraction, ABBE number, hardness, density, and the like, such that the finished lenses contain a distinct phase boundary between the layers. The covalent chemical bonding between the layers at the boundary minimizes problems associated with optical interference, reflection and adhesion which would otherwise be expected.
In the preferred embodiment, the photochromic front layer may be composed of a material which is of a chemical composition beneficial for photochromic dye performance, but may not be optimal for other desired lens properties such as grinding and polishing (e.g., the layer may be relatively soft, but allow for rapid switching of the dye from light to dark and dark to light). The rear layer may be composed of a different material beneficial for properties needed for the body of the lens, for example, a composition which is harder and thus more suitable for grinding and polishing operations, impact strength, tinting, and the like. By manufacturing the composite lenses such that the photochromic dye does not permeate the under layer, the photochromic layer can be concentrated in the outer layer that is beneficial for photochromic dye performance without loss of dye in a layer that is not beneficial for photochromic dye performance.
Gupta et al. U.S. Pat. No. 5,531,940 discloses the initial formation of a coated layer of a photochromic dye-containing casting resin that is in the gel state in a thickness of about 0.025 mm to 1 mm that is subsequently cured while in contact with a finished or semifinished plastic lens preform, with or without a layer of an uncured resin there between. Gupta et al. '940 requires a finished or semifinished plastic lens preform and, optionally, an outer layer of lens material having a higher cross-link density than the lens material carrying the photochromic dye in order to obtain sufficient scratch resistance on the outer surface of the lens, due to the softness of the photochromic dye-containing outer layer. A thin layer of resin is necessary in the Gupta et al. process, which requires a finished or semifinished preform, “to promote rapid curing and decrease the probability of developing stress and distortion in the resulting finished lens”.
It would be highly desirable to manufacture a composite or layered photochromic lens that has an outer photochromic dye-containing polymeric lens layer that has a thickness greater than about 1 mm in order to extend the useful life of the photochromic layer beyond that obtained in the prior art. It also would be highly desirable to manufacture a composite photochromic lens having a relatively thick photochromic dye-containing layer that can be cured relatively quickly without suffering from layer adhesion, stress and distortion problems commonly associated with the manufacture of composite lenses. It would also be desirable to manufacture a lens having the above-described photochromic layer and at the same time provide a distinct boundary between the photochromic and adjacent lens material layer to permit ready verification of the uniformity and thickness of the photochromic layer. These problems have been overcome in accordance with the principles of the present invention, as will become more apparent from the following detailed description of the present invention, wherein (a) a photochromic dye-containing resin, having a thickness of at least 0.4 mm, preferably at least about 0.5 mm, more preferably thicker than about 1 mm, is cured while in contact with (b) a layer of a resin capable of forming an ophthalmic lens, free of a photochromic dye, wherein one of the layers (a) or (b) is cured to the gel state prior to adding the other layer, in a liquid ungelled state, to the mold (without including a finished or semifinished lens preform in the mold), and curing the composite layers, together, using either thermal and/or actinic radiation, e.g., UV, methods, most preferably with actinic radiation.
In accordance with one embodiment of the process of the present invention, a relatively thick layer (at least about 0.4 mm in thickness, preferably about 1 to 2 mm) of a free radical polymerizable monomer or oligomer composition containing one or more photochromic dyes is placed on the surface of a mold used to produce the front surface of the lens. The composition is partially cured to form a soft gel with a degree of hardness, as measured with a Barber Colman type 935 machine of less than 65, preferably less than 60, more preferably less than 40, still more preferably less than 20, most preferably less than 5. The front surface mold containing the photochromic gel is then used in combination with a suitable gasket and a glass mold with appropriate curvature to form a closed assembly into which is cast a polymerizable liquid composition containing no photochromic dye. The photochromic front surface gel layer and the non-photochromic polymerizable composition are then cured while in close contact to form a hard plastic which is then separated from the gasket and molds, resulting in the composite photochromic lens, having exceptional adherence of the layers, which will not delaminate.
In accordance with another embodiment of the process of the present invention, a relatively thick layer of at least about 0.4 mm in thickness, preferably about 1 to 20 mm, of a free radical polymerizable monomer or oligomer composition that does not contain a photochromic dye is placed between the surfaces of two molds of suitable curvature to produce the rear layer of the composite lens. The composition is partially cured to form a soft gel (preferably with a degree of hardness as previously defined). The rear surface mold containing the gel is then used in combination with a suitable gasket and a glass mold with appropriate curvature to form a closed assembly into which is placed a polymerizable liquid composition containing a photochromic dye to form the front, photochromic lens layer. The polymerizable photochromic front surface composition and the non-photochromic gel layers are then cured while in close contact to form a hard plastic lens comprised of both layers with a distinct boundary between them. The formed lens is then separated from the gasket and molds, resulting in the composite photochromic lens, having exceptional adherence of the layers, which will not delaminate.
The resulting lens shown in FIG. 1 overcomes a number of problems previously described. First, the photochromic layer 1 can be relatively thick, allowing for potentially better photochromic lifetime than coated or imbibed lenses. Second, the photochromic layer has a generally uniform thickness which is relatively thin when compared to the thickness of the lens as a whole. As such, the overall darkness and color uniformity is superior to lenses incorporating the photochromic dyes throughout the lens material. Third, because either the photochromic dye-containing layer 1 or the layer containing no photochromic dye 2 is partially polymerized to a gel state when the ophthalmic lens composition forming the other (uncured) layer is introduced into the assembly, and the two layers are then together brought to the cured state, each layer is cured while in contact with the other, and covalent chemical bonding occurs between the two layers resulting in excellent adhesion.